Concern over the depletion and environmental impact of fossil fuels has stimulated strong interest in the development of alternative energy sources. Significant investments in areas such as batteries, fuel cells, hydrogen production and storage, biomass, wind power, algae, and solar energy have been made as society seeks to develop new ways of creating and storing energy in an economically competitive and environmentally benign fashion. The ultimate objective is to minimize society's reliance on fossil fuels and to do so in an economically competitive way that minimizes greenhouse gas production.
A number of experts have concluded that to avoid the serious consequences of global warming, it is necessary to maintain CO2 at levels of 550 ppm or less. To meet this target, based on current projections of world energy usage, the world will need 17 TW of carbon-free energy by the year 2050 and 33 TW by the year 2100. The estimated contribution of various carbon-free sources toward the year 2050 goal are summarized below:
Projected EnergySourceSupply (TW)Wind2-4Tidal2Hydro1.6Biofuels5-7Geothermal2-4Solar600Based on the expected supply of energy from the available carbon-free sources, it is apparent that solar energy is the only viable solution for reducing greenhouse emissions and alleviating the effects of global climate change.
The field of solar energy is currently dominated by solar cells utilizing crystalline silicon as the active photovoltaic material. Crystalline silicon, however, has a number of disadvantages as a solar energy material. First, preparation of crystalline silicon is normally accomplished through a seed-assisted Czochralski method. The method entails a high temperature melting process along with controlled cooling at near-equilibrium conditions and refining to produce a boule of crystalline silicon. Although high purity crystalline silicon can be achieved and the Czochralski method is amenable to n- and p-type doping, the method is inherently slow, expensive, and energy intensive.
Second, as an indirect gap material, crystalline silicon has a low absorption efficiency. Thick layers of crystalline silicon are needed to obtain enough absorption of incident sunlight to achieve reasonable solar conversion efficiencies. The thick layers add to the cost of crystalline silicon solar panels, lead to a significant increase in weight, necessitate bulky installation mounts, and make crystalline silicon solar panels rigid and unsuitable for applications requiring a flexible photovoltaic material.
A number of alternatives to crystalline silicon are currently being pursued in an effort to achieve more versatile, more efficient and less expensive photovoltaic materials. Among the alternatives are CdTe, CdS, and CIGS (Cu—In—Ga—Se alloys). CdTe and CdS suffer from the problem that Cd is a toxic element that requires special disposal procedures. In addition, Te is a relatively scarce element, having an abundance comparable to that of Pt. Although CIGS shows promising solar efficiency, it is difficult to achieve uniform stoichiometry over the large scale substrates required for high-volume manufacturing.
Amorphous silicon (and hydrogenated or fluorinated forms thereof) is an attractive alternative to crystalline silicon. Amorphous silicon is a direct gap material with a high absorption efficiency. As a result, lightweight and efficient solar cells based on thin layers of amorphous silicon and related materials are possible. The instant inventor, Stanford R. Ovshinsky, is the seminal figure in modern thin film semiconductor technology. Early on, he recognized the advantages of amorphous silicon (as well as amorphous germanium, amorphous alloys of silicon and germanium as well as doped, hydrogenated and fluorinated versions thereof) as a solar cell material and was the first to recognize the advantages of nanocrystalline silicon as a photovoltaic material. Stanford R. Ovshinsky also pioneered the automated and continuous manufacturing techniques needed to produce thin film, flexible large-area solar panels based on amorphous, nanocrystalline, microcrystalline, polycrystalline or composite semiconductors
Representative discoveries of Stanford R. Ovshinsky in the field of amorphous semiconductors and photovoltaic materials are presented in U.S. Pat. No. 4,400,409 (describing a continuous manufacturing process for making thin film photovoltaic films and devices); U.S. Pat. No. 4,410,588 (describing an apparatus for the continuous manufacturing of thin film photovoltaic solar cells); U.S. Pat. No. 4,438,723 (describing an apparatus having multiple deposition chambers for the continuous manufacturing of multilayer photovoltaic devices); U.S. Pat. No. 4,217,374 (describing suitability of amorphous silicon and related materials as the active material in several semiconducting devices); U.S. Pat. No. 4,226,898 (demonstration of solar cells having multiple layers, including n- and p-doped); U.S. Pat. No. 5,103,284 (deposition of nanocrystalline silicon and demonstration of advantages thereof); and U.S. Pat. No. 5,324,553 (microwave deposition of thin film photovoltaic materials ) as well as in articles entitled “The material basis of efficiency and stability in amorphous photovoltaics” (Solar Energy Materials and Solar Cells, vol. 32, p. 443-449 (1994); and “Amorphous and disordered materials—The basis of new industries” (Materials Research Society Symposium Proceedings, vol. 554, p. 399-412 (1999).
Current efforts in photovoltaic material manufacturing are directed at increasing the deposition rate. Higher deposition rates lower the cost of thin film solar cells and lead to a decrease in the unit cost of electricity obtained from solar energy. As the deposition rate increases, thin film photovoltaic materials become increasingly competitive with fossil fuels as a source of energy. Presently, PECVD (plasma-enhanced chemical vapor deposition) is the most cost-effective method for the commercial-scale manufacturing of amorphous silicon and related solar energy materials. Current PECVD processes provide uniform coverage of large-area substrates with device quality photovoltaic material at a deposition rate of ˜2-3 Å/s and work directed at new processes with deposition rates of 10 Å/s and above is in progress.
In order to improve the economic competitiveness of plasma deposition processes, it is desirable to increase the deposition rate. Increases in deposition rate require strategies for overcoming basic limitations associated with current plasma deposition techniques. One problem with current plasma-deposited photovoltaic materials is the presence of a high concentration of intrinsic defects in the as-deposited state. The intrinsic defects include structural defects (e.g. dangling bonds, strained bonds, unpassivated surface states, non-tetrahedral bonding distortions, coordinatively unsaturated silicon or germanium) that create electronic states within the bandgap of the photovoltaic material. The midgap states detract from solar conversion efficiency by acting as nonradiative recombination centers that deplete the concentration of free carriers generated by absorbed sunlight. Instead of being available for external current, the energy of many of the photoexcited free carriers is dissipated thermally through nonradiative decay. The external current delivered by a photovoltaic material is reduced accordingly.
The intrinsic defects are also believed to contribute to a degradation of solar cell performance of amorphous silicon-based photovoltaic materials through the Staebler-Wronski effect. The Staebler-Wronski effect is a photo-induced degradation of the performance of amorphous silicon and related materials (e.g. hydrogenated, fluorinated or doped forms thereof) that is responsible for a decrease in solar efficiency of up to 25%. Although the origin of the Staebler-Wronski effect has not been definitively established, it is believed to be caused in part by the relatively high diffusion coefficient of hydrogen and the changes in local bonding coordination promoted by hydrogen.
One strategy for reducing the concentration of intrinsic defects in plasma-deposited photovoltaic materials is to include species in the plasma that provide defect compensating agents that can be incorporated into the deposited film. Inclusion of excess hydrogen in the plasma, for example, leads to a marked improvement in the quality of the material and the ability to make nanocrystalline phases in amorphous silicon by promoting the incorporation of hydrogen as a defect compensating agent in the film. Compensating agents passivate defects, saturate bonds, relieve bond strain and remove non-tetrahedral structural distortions that may occur in as-deposited thin film materials. As a result, the concentration of midgap band states is reduced and higher solar conversion efficiency is achieved.
Recognizing that the use of excess H2 leads to poor gas utilization, incomplete passivation of defects, and the generation of various undesirable species in the plasma, S.R. Ovshinsky has advocated the use of fluorine as a compensating agent. In particular, S.R. Ovshinsky has shown that the inclusion of fluorine provides more regular bonding, leads to fewer defects, and enables deposition of nanocrystalline phases. (See U.S. Pat. No. 5,103,284 (formation of nanocrystalline silicon from SiH4 and SiF4); U.S. Pat. No. 4,605,941 (showing substantial reduction in defect states in amorphous silicon prepared in presence of fluorine); and U.S. Pat. No. 4,839,312 (presents several fluorine-based precursors for the deposition of amorphous and nanocrystalline silicon)).
S.R. Ovshinsky has further shown that the Staebler-Wronski effect is greatly diminished in nanocrystalline silicon. Through formation of nanocrystalline silicon or a nanocrystalline phase within an amorphous silicon matrix, photodegradation and light soaking effects are greatly ameliorated. Nanocrystalline silicon provides sufficient order to minimize intrinsic defects while providing enough structural flexibility to avoid the disadvantages associated with the rigid lattice of crystalline silicon. As a result, nanocrystalline silicon provides high carrier mobility, high photocurrents, and a stable photovoltaic response.
In addition to forming materials having a low concentration of defects, it is desirable to deposit materials that exhibit favorable electronic properties. In photovoltaic materials, for example, a high collection efficiency of photo-excited carriers is desired. When incident sunlight penetrates and is absorbed by a photovoltaic material, it creates photo-excited carriers in the interior of the material by promoting electrons from the valence band to the conduction band. In order to harness electrical energy from the photovoltaic material, it is necessary for the photo-excited carriers to move through the material to an electrical contact placed at the surface of the material. The ability of photo-excited carriers to reach surface contacts depends on the concentration of defects within the material and on the transport properties of the carriers. Carrier mobility is a measure of the ease with which carriers can migrate spatially within a material and carrier collection is facilitated if the mobility of electrons and holes is high. Excess photo-excited carrier is trapping, recombination, or other dissipation leads to low carrier mobility and reduces the likelihood that carriers are extracted from the material to contribute to an external current.
A drawback of amorphous silicon as a solar energy material is its low hole mobility. The low hole mobility reduces solar efficiency and makes simple p-n junction devices based on amorphous silicon impractical. Instead, inclusion of an intrinsic layer and more complicated p-i-n structures are needed to compensate for poor hole mobility.
An alternative strategy for overcoming the poor hole mobility of amorphous silicon was invented by S. R. Ovshinsky in U.S. Pat. No. 5,103,284 (the '284 patent). The '284 patent describes the formation of a clustered semiconductor material, with dimensions on the order of 10-50 Å (and up to 100 Å), having a state of order intermediate between the extended periodic lattice structure of crystalline silicon and the random, disordered structure of amorphous silicon.
Ovshinsky showed that the intermediate range order nature of the ordered cluster material represents a new state of matter having properties that combine features of the amorphous and crystalline phases. In particular, Ovshinsky showed that the ordered cluster material possesses the high carrier mobility of crystalline silicon and the high absorption efficiency of amorphous silicon. Ovshinsky has also proved by direct observation that the intermediate range order state is a new form of order that is two-dimensional in character without grain boundaries and the defects that accompany them.
Within the size range of the ordered cluster material of the '284 patent, Ovshinsky showed that the bandgap becomes more direct and the absorption coefficient increases as size decreases. The dimensions of the ordered cluster material are in the quantum regime, a new regime of order in which selection rules are relaxed and different principles govern electronic properties. In the quantum regime, the bandgap is more direct and the photovoltaic response of the material is more stable because the factors responsible for the Staebler-Wronski effect are absent.
Ovshinsky has further shown that fluorine promotes the formation of intermediate range order material and that fluorine is particularly advantageous as a component of thin film photovoltaic materials because of its ability to eliminate surface states that serve as traps for photo-excited carriers.
Ovshinsky has also been a pioneer in developing diagnostic methods, including Raman spectroscopy, for detecting ordered clusters and has elucidated the concept of intermediate range order as a signature for high quality photovoltaic materials. Although the ordered cluster material has desirable properties, current techniques for forming it suffer from low deposition rates.
A need exists in the art for a method for preparing photovoltaic materials (including amorphous, nanocrystalline, microcrystalline, and polycrystalline forms of silicon, germanium, and alloys of either) at high deposition rates without sacrificing photoexcitation efficiency, carrier collection efficiency, and other attributes of quality. The low deposition rates needed to achieve high efficiency photovoltaic materials through conventional plasma deposition techniques limit the economic competiveness of photovoltaic materials relative to fossil fuels and motivate a search for new deposition processes.